Low-pressure mercury lamps have been widely used as light sources of water treatment apparatuses for, for example, performing so-called sterilization to decompose organic materials contained in water or to kill germs contained in water (see, for example, Patent Literatures 1 and 2).
However, when a low-pressure mercury lamp is used as the light source of a water treatment apparatus, the low-pressure mercury lamp is often placed in water to be treated. A problem that occurs under such use conditions is that the light-emission characteristics of the low-pressure mercury lamp are greatly affected by the temperature of water.
The reason that such a problem occurs is that the low-pressure mercury lamp contains mercury as a light-emitting material and the vapor pressure of mercury must reach a prescribed level during operation in order to obtain a favorable light-emitting state. When the temperature of water is extremely low, mercury is not sufficiently vaporized, and this causes a problem in that sufficient light emission is not obtained.
Recently, in view of such circumstances, so-called mercury-free lamps that do not use mercury as a light-emitting material receive attention as light source lamps constituting light sources of water treatment apparatuses. For example, a lamp using xenon as a light-emitting material has been proposed (see, for example, Patent Literature 3).
Specifically, Patent Literature 3 discloses, as a light source lamp of a water treatment apparatus, a discharge lamp including: a light-emitting tube made of a light-transmitting dielectric material such as quartz glass; and electrodes (external electrodes) disposed on the outer surface of the light-emitting tube (this discharge lamp may be hereinafter referred to as an “external electrode discharge lamp”).
However, in this external electrode discharge lamp, the electrodes to which high voltage is applied are located outside and exposed, and this causes various problems depending on use environment conditions.
Specifically, when an external electrode discharge lamp is used as the light source of a water treatment apparatus and disposed in water to be treated, the external electrodes come into contact with water, so corrosion may occur in the external electrodes. Corrosion of the external electrodes causes not only a problem in that the external electrodes themselves wear and therefore adversely affect discharge but also a problem in that corrosion products falling from the surfaces of the corroded external electrodes are mixed into the water to be treated. Another problem is that since the external electrodes are exposed to the outside of the light-emitting tube, an electric leakage or an electric shock may occur, so that sufficient safety cannot be obtained.
These problems not only occur when the external electrode discharge lamp is used as the light source of a water treatment apparatus but also occur similarly when the external electrode discharge lamp is used as the light source of, for example, an air sterilization apparatus because water vapor is present in air.
In some types of external electrode discharge lamps, a light-emitting tube including electrodes (external electrodes) disposed on its outer surface is disposed inside a wrapper tube. In the external electrode discharge lamp having the above configuration, the external electrodes do not come into contact with water even when the lamp is placed in water. However, when air is present inside the wrapper tube, micro discharge may occur in the gaps between the light-emitting tube and the external electrodes, and therefore ozone may be generated. This causes a problem in that harmful effects such as corrosion of the electrodes occur.
One possible solution to the above problems is to dispose the pair of electrodes inside the light-emitting tube.
Excimer lamps having a configuration in which a pair of electrodes are disposed inside a light-emitting tube have been proposed as one type of mercury-free lamp (see, for example, Patent Literatures 4 and 5).
More specifically, Patent Literature 4 discloses an excimer lamp including: a cylindrical light-emitting tube having sealed portions at both ends; and metal rods constituting a pair of electrodes and disposed such that one ends of the rods are located inside the light-emitting tube and the other ends protrude outward from sealed portions and extend therefrom. In at least one of the metals rods constituting the electrodes of the above excimer lamp, the surface of the one end located inside the light-emitting tube and functioning as an electrode is covered with a dielectric layer made of a dielectric material.
Patent Literature 5 discloses an excimer lamp including: a light-emitting tube having a cylindrical outer tube that is made of a dielectric material and has closed opposite ends; and two cylindrical inner tubes that are made of a dielectric material and have open opposite ends. In the light-emitting tube of this excimer lamp, the two inner tubes extend along the tube axis of the outer tube, and the openings of the inner tubes are disposed so as to protrude outward from the ends of the outer tube. A light-emitting space is formed by the inner surface of the outer tube and the outer surfaces of the inner tubes located inside the outer tube. In this excimer lamp, a metal rod constituting one of electrodes is disposed inside one of the inner tubes, and a metal rod constituting the other electrode is disposed inside the other inner tube.
The excimer lamp disclosed in Patent Literature 4 has a configuration in which the surface of an electrode is covered with a dielectric layer. However, this excimer lamp has a problem in that, particularly when glass is used as the dielectric material and the dielectric layer is interposed between the light-emitting tube and the metal rod constituting the electrode at a seal portion of the light-emitting tube, the light-emitting tube may break at the seal portion because of the difference in thermal expansion coefficient between the material of the metal rod and the dielectric material. This problem also occurs in a seal portion of the light-emitting tube when the light-emitting tube and a metal rod constituting an electrode are in direct contact with each other and the light-emitting tube is made of the same material as the dielectric material (the material constituting the dielectric layer).
The excimer lamp disclosed in Patent Literature 5 has a configuration in which the electrodes are disposed inside the inner tubes having at both ends openings in communication with the outside. This excimer lamp has a problem in that, when the electrodes (metal rods) are spaced apart from the inner surfaces of the inner tubes, the capacitance of the inner tubes becomes small.
To prevent oxidation of the electrodes (metal rods) disposed inside the inner tubes, the inside of each inner tube must be reduced in pressure or have an inert gas atmosphere. For example, it is conceivable that an inert gas is caused to flow inside the inner tubes when the lamp emits light as shown in Patent Literature 5. Alternatively, it is conceivable that opposite ends of the inner tubes are sealed to form sealed spaces inside the inner tubes, and the sealed spaces are reduced in pressure or filled with an inert gas. However, the following problems occur in these cases.
To cause the inert gas to flow inside the inner tubes, means for causing the inert gas to flow must be provided separately. To seal the opposite ends of the inner tubes, hermetic seal structures must be formed by bringing the metal rods constituting the electrodes into intimate contact with the ends of the inner tubes. Therefore, particularly when glass is used as the dielectric material constituting the inner tubes, a problem occurs in that the seal portions may break because of the difference in thermal expansion coefficient between the material of the metal rods and material of the inner tubes (the dielectric material).